Method For Controlling a Hydraulic Actuator Comprising a Rapid Drain Valve and a Control System and a Friction Coupling Comprising an Actuator of this Type

ABSTRACT

A method of controlling a hydraulic actuator of a friction coupling that includes a pump, which is driven by an electric motor, a pressure line, which contains a non-return valve and which runs from the pump to an actuator cylinder with an actuator piston that acts on the friction coupling. A rapid drain valve has a flow connection to the actuator cylinder and contains a slide that responds to the pressure prevailing on the side of the pump that faces the slide. To optimize the dynamic and static control behavior of the actuator, a control variable is determined for the electric motor from the target pressure and the actual pressure in the actuator cylinder. At least two different control algorithms are executed, depending on whether the difference between the target pressure and the actual pressure is positive or negative.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/AT2005/000444, filed Nov. 8, 2005, and which claims the benefit ofAustrian Utility Model Application No. GM 805/2004, filed Nov. 8, 2004.The disclosures of the above applications are incorporated herein byreference.

FIELD

The invention relates to a method for controlling a hydraulic actuatorof a friction clutch which comprises a pump driven by an electric motorcontrolled by a control system, a pressure line including a check valveand running to an actuator cylinder having an actuator piston, with thepressure in the actuator cylinder having to be controlled or feedbackcontrolled, and a fast drain valve including a slider responsive to thepressure prevailing at the side of the pump facing it. In thisconnection, in particular the actuator of a multiple-disk clutch in thedrivetrain of a motor vehicle is being thought of, on which particulardemands are made due to the special characteristics of such clutches andto the special demands in motor vehicles with driving dynamic systems.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

The special demands on the controllability of friction clutches arepresent both with respect to the precision of the setting of a specifictorque and with respect to the speed of the control. The latter inparticular on the release of the clutch, for instance on an ABSintervention or an ESP intervention. Furthermore, the electric motorshould use as little energy as possible over all, that is it should alsoonly run when necessary. There is also the demand for intrinsic safety.This means that the most secure state (usually that is the releasedclutch) should be adopted automatically in the event of system failure.

These demands also require an embodiment of the actuator in accordancewith the preamble of the first claim, such as is, for example, thesubject matter of WO 2004/040158 A2 of the applicant. Further detailscan be seen from this. An actuator of this type is cost-effectivebecause the control valves required with conventional actuators can bedispensed with. The control of the electric motor for the actuation ofthe actuator is, however, demanding from a technical control viewpointand is the subject of the present invention, which can be usedindependently of the specific construction and of the control of theelectric motor itself.

SUMMARY

The object underlying the invention is thus to teach a method and acontrol system that permits the precise setting of a specific pressure,the very fast lowering of the pressure and the maintaining of thepressure using a minimum of electrical energy and is moreoverintrinsically safe. The latter means that the pressure reliably falls ona failure of the control.

This is achieved in accordance with the invention in that a controlvariable for the electric motor is determined from the desired pressureand the actual pressure in the actuator cylinder, with at least twodifferent control algorithms being carried out in dependence on the signof the difference of the desired pressure and actual pressure. Thecontrol variable for the electric motor depends on its specificconstruction and control. It can be a permanently excited direct currentmotor with control of the current strength or voltage or any othercontrollable electric motor. The sign of the difference is to beunderstood as the sign preceding it. It is positive when the desiredpressure is larger than the actual pressure and negative in the reversecase. It is zero when the pressure difference is smaller than apredetermined tolerance, with this also being able to be preset by ahigher level system (for example a driving dynamics controller). Thedifferent control algorithms first permit a precise setting of aspecific pressure with a positive sign and an extremely fast lowering ofthe pressure with a negative sign, and also additional measures tomaintain the respective pressure in as energy saving a manner aspossible. This takes the fact into account that the control path in thetwo operating states has a different structure and behaves differentlydue to the interaction of the check valve and the fast drain valve.

To build up the pressure with a positive sign, the control algorithmcompares the desired pressure in the actuator cylinder with the actualpressure and forms a control variable for the electric motor. Thecontrol parameters are adapted in dependence on operating parameters, inparticular in dependence on the pressure in the actuator cylinder. Thecontrol algorithm is preferably that of a PID control; however, it canalso be that of a state control or fuzzy logic. The control parametersof the controller are to be selected accordingly to match the propertiesof the control path comprising, on the build of pressure, electricmotor, pump, check valve, pressure cylinder and friction clutch. Theadaptation takes the fact into account that the package stiffness of thewhole clutch (in other words: the spring characteristic) is highlynon-linear over the closing path of the clutch. It breaks down intothree part regions having greatly differing gradients.

In a further development of the control algorithm on the build up ofpressure (positive sign), it is that of a cascade controller, with adesired speed of the electric motor being determined in a firstcontroller from the difference of the desired pressure and actualpressure in the actuator cylinder, a desired electrical parameter beingdetermined in a second controller from the difference of the desiredspeed of rotation and the actual speed of rotation of the electricmotor, and a control variable with which the electric motor iscontrolled being determined in a third controller from the difference ofthe desired electrical parameter and the actual electrical parameter.

The cascading has the following advantages: more favorable dynamicsbecause the time constants of the individual controllers can be adaptedto the respective time constants of the control path; better controlelimination of variable disturbance due to the internal feedback;protection of the electric motor from overloading. A further improvementis achieved with the cascading in that the control parameters of thefirst controller are adapted in dependence on operating parameters, inparticular on the pressure in the actuator cylinder. Instead of theadaptation, a plurality of controllers with different control parametersand a subsequent selection can also be used.

For the pressure reduction (negative sign), the control algorithm forms,in a first variant, a control variable for the electric motor by acomparison of the desired position of the slider of the fast drain valvewith its actual position, with the desired position of the sliderprimarily being formed from the values of the desired pressure and theactual pressure in the actuator cylinder. In this connection, the actualposition of the slider is determined from one or more operatingparameters of the actuator, for instance from a parameter correspondingto the angle of rotation of the electric motor. The position of the fastdrain valve can, however, also be measured.

For the pressure reduction (negative sign), the control algorithm forms,in a second variant, a control variable for the electric motor by acomparison of the desired gradient with the actual gradient of thepressure in the actuator cylinder, with the desired gradient beingformed as a function of the desired pressure and the actual pressure inthe actuator cylinder by a time derivation of the actual pressure in theactuator cylinder.

In a further development of the control algorithm on the reduction ofpressure (negative sign), it is that of a cascade controller, with adesired speed of the electric motor being determined in a firstcontroller from the difference of the desired position and the actualposition of the electric motor, a desired electrical parameter beingdetermined in a second controller from the difference of the desiredspeed of rotation and the actual speed of rotation of the electricmotor, and a control variable with which the electric motor iscontrolled being determined in a third controller from the difference ofthe desired electrical parameter and the actual electrical parameter.The aforesaid advantages of a cascade control are also utilized againhere.

In a further development of the method in accordance with the invention,special measures are also to be provided for the maintenance of thepressure in the actuator cylinder (when the sign of the difference ofthe desired pressure and actual pressure is within a predeterminedtolerance). In a first variant, the control algorithm then monitors theactual pressure in the actuator cylinder and forms, with a definedpressure drop, a control variable for the electric motor whichaccelerates it from a reduced speed or sets it in motion when at astandstill. In a second variant, the control algorithm monitors theposition of the slider and forms a control variable for the electricmotor on a defined deviation occurring. In this connection, the controlvariable for the electric motor is the motor current. If the pressure inthe actuator cylinder should be maintained, only the fast drain valvehas to be kept closed. The pressure required for this is determined bythe force of the spring acting on the slider and a specific motorcurrent corresponds to this pressure.

The invention also relates to a system for the control of a hydraulicactuator of a friction clutch, said hydraulic actuator including thecomponents listed in the preamble of the first claim, with the systemcontaining a processor and a driver stage for the control of theelectric motor. It is characterized in that the processor forms at leasttwo controllers with different control behaviors and contains aselection logic which selects the output signal of the one or the othercontroller in dependence on whether the pressure in the actuatorcylinder should be raised or lowered. This takes the fact into accountthat the control path in the two operating states has a differentstructure and behaves differently due to the interaction of the checkvalve and the fast drain valve. A specific pressure can thus both be setprecisely and be lowered very fast again with an overall minimalconsumption of electrical energy.

In a further development of the system in accordance with the invention,the one and/or the other controller is made as a cascade controller,with, in the cascade, a first controller comparing the respectivecontrol parameters with one another and forming a desired speed for theelectric motor, a second controller comparing the desired speed with theactual speed of the electric motor and forming a desired electricalparameter, and a third controller comparing the desired electricalparameter with an actual electrical parameter and determining a controlvariable with which the electric motor is controlled. The advantages ofthe cascade control listed further above are thus achieved, with theincreased effort on the implementation in a processor only consisting ofadditional measurement devices for the operating parameters fed back inthe internal loops or with the measurement devices anyway being present.

The invention also relates to a friction clutch for the drivetrain of amotor vehicle comprising an actuator and which has a control system withthe torque transmissible by the friction clutch being substantiallyproportional to the pressure in the actuator cylinder.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

FIG. 1 illustrates a scheme of the actuator in accordance with theinvention with a friction clutch;

FIG. 2 is a block diagram of the control system in accordance with theinvention;

FIG. 3 illustrates a scheme of the control for a positive sign;

FIG. 4 illustrates a variant of the controller of FIG. 3;

FIG. 5 illustrates a scheme of the controller for a negative sign in afirst embodiment;

FIG. 6 illustrates a variant of the controller of FIG. 5;

FIG. 7 illustrates a scheme of the controller for a negative sign in asecond embodiment.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

In FIG. 1, a cylinder in piston unit is designated in summary by 1, avalve unit by 2 and an electric motor and pump unit by 3. A pressurespace 4 is present in the cylinder in piston unit 1 and is incommunication via a line 6 with the valve unit 2, with the pressurefluid contained in the pressure space 4 acting on a piston 5. Thispiston 5 is part of a friction clutch 7 or is directly in communicationtherewith. The friction clutch 7 is only indicated since it is of theusual construction with disks and a spring. In the friction clutch 7,the pressure exerted by the piston 5 acts against the force of thisspring and of the clutch disks. As the pressure increases, the torquetransmissible by the clutch increases approximately proportionally withthe pressure.

The valve unit 2 contains a fast drain valve 8 and a check valve 9. Thelatter has a ball 9′ pressed toward a seat by a spring 9″. The fastdrain valve 8 is formed by a socket 10 having at least one opening 11,which opening is in communication with the pressure space 4 via the line6, and by a piston 12 displaceable in the socket 10. The piston 12separates a first space 13 containing a compression spring 14 from asecond space 17. The first space 13 is in communication via a drain line15 with a sump 16 from which the electric motor and pump unit 3 sucks influid and into which it pumps fluid. A pressure line 18 is connected tothe second space 17 and in turn establishes the connection between theelectric motor and pump unit 3 and to the pressure space 4 via the checkvalve 9.

The electric motor and pump unit 3 comprises a pump for the pressurefluid and a motor 20 which is controlled by a control system 21. In theembodiment described, a permanently excited DC motor is used. As theinput signal, the control system 21 receives actual values determined bysensors 22 (only a pressure sensor is indicated here) and, via a line23, a desired value of a pressure in the actuator cylinder whichgenerates the contact pressure acting on the disks of the clutch 7 andcorresponds to the maximum torque to be transmitted by the clutch. Thepreviously described elements form the actuator of the clutch 7.

The manner of operation of the described arrangement is as follows: Inthe position shown in FIG. 1, the electric motor and pump unit 3 eitherdoes not pump at all or at a pressure which is not sufficient to openthe check valve 9 or to close the fast drain valve 8. No pressure ispresent in the pressure space 4; the clutch, which is not shown, is thusnot acted on, that is does not transmit any torque. If the pressure ofthe pressure medium in the line 18 delivered by the pump 19 nowincreases, this acts in the second space 17 on the lower side of theslider 12 made as a piston against the force of the spring 14. At aspecific pressure, the slider 12 starts to move upwardly, with itclosing the opening 11 and thus the outflow from the pressure space 4.Only when the opening 11 is fully closed does the check valve 9 open andcan pressure fluid flow into the pressure space 4 and control the clutchaccordingly.

If the pump 19 is now stopped, the pressure acting on the slider 12drops; the check valve 9 closes at the same time. The slider 12 isslowly pressed downwardly by the spring 14 (depending on the leakage ofthe pump), whereby the openings 11 become free again after a specifictime and the pressure fluid can escape from the pressure space 4 intothe sump 16. If the electric motor and pump unit 3 is now switched oversuch that the pumping direction also reverses, that is the pump 19 pumpsout of the pressure line 18 into the sump 16, an underpressure arisesunder the slider 12 and substantially accelerates its downward movement.Then, on switching over of the motor 20, the clutch is fully opened fora moment as is required, for example, in the case of ABS braking.

If the pressure space 4 is under pressure and the electric motor andpump unit 3 maintains the fast drain valve closed, the pressurecontinues to be maintained for a while with a good seal. This meansthat, in steady state operation with an engaged clutch, the electricmotor and pump unit 3 only has to maintain the pressure for the sliderto remain closed. The output amount is almost zero since leakage mainlytakes place in the interior of the pump. A substantial saving in energyis thus achieved.

In FIG. 2, the total control system 21 is shown as part of a feedbackcontrol circuit which it forms with an actuator and its control pathwhich are here indicated together and designated by 28. Various sensorsare attached to the actuator and to the control path and generatesignals 22, and indeed:

-   22 a: actual pressure (p_(act)) in the actuator cylinder 4;-   22 b: actual current strength (I_(act)) of the current supplied to    the electric motor 20;-   22 c: actual voltage (U_(act)) of the current supplied to the motor    20;-   22 d: actual angle of rotation of the motor 20;-   22 e: actual speed of the motor 20,-   22 f: the actual position (X_(act)) of the slider 12;-   22 g: the position of the actuator piston 5;-   22 h: a signal corresponding to the pressure signal (for example,    determined from other signals, for instance from a torque signal or    rotational speed signal).

The actual pressure signal 22 a or 22 h in any case and individual onesof the further signals 22 b to 22 g are available to the control system21, in the same way as a signal 23 which is emitted by a higher levelcontrol system, which indicates the desired pressure (p_(des)) in theactuator cylinder 4 and which is substantially proportional to themaximum torque to be transmitted by the clutch.

The control system 21 comprises in general terms an analog/digitalconverter 25 which makes the signals 22 and 23 available to a computingunit 26 in digital form. The latter's output signal 36 is a controlvariable for the motor 20 which is supplied to a driver stage 27 whichcontrols electrical current supplied to the electric motor with respectto voltage and/or current strength. The input signal 23 can already bepresent in digital form and also additionally contain the width of thetolerance range.

Three controllers 30, 31, 32 and a selection logic 33 are provided inthe computing unit 26. All three are arranged in parallel, they receive,as input parameters, the desired pressure 23 (p_(des)) and the measuredsignals 22, but at least the actual pressure 22 a or 22 h, and all threecontrollers provide, as the output signal, a control variable 34 a, 34b, 34 c for the electric motor 20, from which the selection logic 33selects a signal 36, likewise in dependence on the desired pressure 23(p_(des)) and the measured signals 22, but at least on that of theactual pressure 22 a or 22 h (p_(act)). The three controllers 30, 31, 32connected in parallel come into effect, only one in each case, indifferent control situations. The first controller 30, when the desiredpressure 23 (P_(des)) is larger than the actual pressure 22 a or 22 h(p_(act)), that is when the pressure should increase in the actuatorcylinder (and the clutch should be engaged). The sign of the pressuredifference designates its preceding sign, which is positive in thiscase. The second controller 31 acts when the pressure difference, andthus the sign, is negative, which corresponds to a dropping pressure inthe actuator cylinder (and a disengagement of the clutch). Finally, athird controller can be provided to maintain the pressure. It acts whenthe desired pressure and the actual pressure are within the presettolerance. It is also called a maintaining controller. The controlvariable 36 for the motor 20 selected by the selection logic 33 isforwarded to the driver stage 27.

In FIG. 3, the first controller 30 comprises the actual controller 37and a computer 38 for the calculation of the control parameters on thebasis of individual input signals 22, in particular, but notexclusively, of the actual pressure signal 22 a or 22 h. The controlparameters calculated by it (with a PID controller, that is the factorsdetermining the P, I and D functions) are provided to the actualcontroller 37 for adaptation. The controller 30 is thus adaptiveoverall. This takes into account the fact that the relationship betweenthe pressure to be overcome by the actuator piston (5) (comprising theforce of the clutch springs and the contact pressure of the clutch disksrequired for the transmission of a specific torque) and its path isstrongly non-linear. Without the adaptation function, the positioningprocedure in the range of low force would take much too long. Thefactors describing the controller (P, I, and D factors) are thereforeset in accordance with the input signals 22 and 23, in particular inaccordance with the actual pressure 22 a, 22 h, such that the adjustmentof the piston 5 corresponds to the demands on the dynamics in allranges. The parameters determined by the computer 38 are forwarded tothe actual controller 37 via the connection 39.

When a controller, a connection or a loop are spoken of in the totaldescription, a program module is meant, when a processor is used, whichcarries out the corresponding control algorithm.

In the variant of FIG. 4, the controller 30′ is made as a cascadecontroller which comprises three sub-controllers 40, 43, 45 which areconnected in cascade. The first sub-controller 40 is divided into threeregions 40′, 40″, 40″′ with different control parameters, as analternative solution to the adaptive controller 30 of FIG. 3. It isfollowed by a selection logic 41 which, like the input of the controller40, receives the actual pressure signal (p_(act)) via the “line” 42; it(42) forms an outer return loop of the cascade. The output signal of thefirst sub-controller controller 40 is a desired speed of the motor(n_(des)). The second sub-controller 43 of the cascade is a speedcontroller to which the desired speed signal (n_(des)) of the firstsub-controller 40 and, via a middle return loop 44, an actual speed ofrotation (n_(act)) of the motor is supplied. The output signal is adesired current signal (I_(des)) for the motor. It is compared in thethird sub-controller 45 with the actual current (I_(act)) of the motorand generates a control variable 34 a for the motor 20. The actuator andthe control path 28 are indicated.

FIG. 5 shows the second controller 31 for a negative sign (pressuredrop) in a first embodiment. An actual value 22 f (x_(act))corresponding to the actual position of the slider 12 and a desiredvalue of the position of the slider 12 (x_(des)) are supplied to theactual controller 50. Said desired value is primarily calculated fromthe desired pressure 23 (p_(des)) in the actuator cylinder and from theactual pressure signal 22 a or 22 h (Pact). Further measured signals 22can be supplied to the controller via the loop 52. The output signal 34of the actual controller 50 is in turn a control variable 34 b for theelectric motor.

In the variant of FIG. 6, the controller 31 for a negative sign is againmade as a cascade controller. The computing unit 60 determines thedesired position of the slider 12 (x_(des)) from the actual pressure 22a or 22 h (p_(act)) and the desired pressure 23 (p_(des)) in theactuator cylinder 4, with the desired value (x_(des)) of the position ofthe slider 12 being a function of the throughflow cross-section of theopening 11. In a first sub-controller 63, a desired speed of the motor(n_(des)) is determined from the desired value (x_(des)) of the positionof the slider 12 and from its actual value (x_(act)), which isdetermined in a computing unit 61 from the signals 22, preferably fromthe actual angle of rotation 22 d, actual speed of rotation (n_(act)) ofthe motor. The actual position (x_(act)) of the slider 12 is supplied tothe first sub-controller 63 (a position controller) via an externalreturn loop 62. In a second sub-controller 65 (a speed controller), adesired current (I_(des)) for the motor is calculated from the desiredspeed of rotation (n_(des)) of the motor and an actual speed of rotation(n_(act)) of the motor communicated via a middle return loop 64. Thisdesired current (I_(des)) is in turn compared with the actual current(I_(act)) supplied via an internal return loop 66 and a control variable34 b for the electric motor is determined from this in a thirdsub-controller 67 (a current controller).

FIG. 7 shows a second embodiment of the second controller 31 (negativesign) which differs from that of FIG. 5 in that, instead of the desiredposition (X_(des)) of the slider 12, the pressure gradient (dp/dt) isused as the input parameter. The actual controller 70 compares a desiredvalue (dp/dt_(des)) of the pressure gradient with an actual value(dp/dt_(act)) of the pressure gradient. The first is calculated in acomputing unit 71 from the desired pressure 23 (p_(des)) and the actualpressure 22 a or 22 h (p_(act)) in the actuator cylinder 4. The secondis determined in a unit 72 from the actual pressure signal 22 a or 22 h(p_(act)). The output value is again the control variable 34 b for theelectric motor.

The description is merely exemplary in nature and, thus, variations thatdo not depart from the gist of the present disclosure are intended to bewithin the scope of the invention. Such variations are not to beregarded as a departure from the spirit and scope of the presentdisclosure.

1.-16. (canceled)
 17. A method for controlling a hydraulic actuator of afriction clutch (7), which comprises: a) a pump (19) driven by anelectric motor (20) that is controlled by a control system (21); b) apressure line (18), which contains a check valve (9) and which runs fromthe pump (19) to an actuator cylinder (4) having an actuator piston (5)acting on the friction clutch (7), with a pressure in the actuatorcylinder (4) to be controlled; and c) a fast drain valve (8), which isin flow communication with the actuator cylinder (4) and includes aslider (12) responsive to the pressure prevailing at the side of thepump (19) facing it; the method comprising: determining a controlvariable (34 a, 34 b, 34 c) for the electric motor (20) from a desiredpressure (23) and an actual pressure (22 a) in the actuator cylinder(4); and executing at least two different control algorithms (30, 31,32) depending on a sign of a difference between the desired pressure andactual pressure.
 18. A method in accordance with claim 17, wherein, witha positive sign, the control algorithm (30) forms a control variable(34) for the electric motor (20) from the desired pressure (23) and theactual pressure (22 a; 22 h) in the actuator cylinder (4) and itscontrol parameters are adapted in dependence on operating parameters (22b, 22 c, 22 d, 22 e, 22 f, 22 g).
 19. A method in accordance with claim18, wherein the control parameters of the control algorithm (30) areadapted in dependence on the pressure (22 a; 22 h) in the actuatorcylinder (4).
 20. A method in accordance with claim 17, wherein, with apositive sign, the control algorithm (30) is that of a cascadecontroller, with a desired speed (n_(des)) of the electric motor (20)being determined in a first sub-controller (40) from the desiredpressure (23) and the actual pressure (22 a; 22 h) in the actuatorcylinder (4), a desired electrical parameter (I_(des)) being determinedin a second sub-controller (43) from the desired speed (n_(des)) and theactual speed (n_(act)) off the electric motor, and a control parameter(34) with which the electric motor is controlled being determined in athird sub-controller from the desired electrical parameter (I_(des)) andthe actual electric parameter (I_(act)).
 21. A method in accordance withclaim 20, wherein the control parameters of the first sub-controller(40) are adapted in dependence on operating parameters (22 a to 22 g),in particular on the pressure in the actuator cylinder (22 a; 22 h). 22.A method in accordance with claim 17, wherein, with a negative sign, thecontrol algorithm (31) forms a control variable (34) for the electricmotor (20) by a comparison of the desired position (x_(des)) of theslider (12) of the fast drain valve (8) with its actual position(x_(act)), with the desired position of the slider (12) being formedfrom the desired pressure (p_(des)) and the actual pressure (p_(act)) inthe actuator cylinder (4).
 23. A method in accordance with claim 22,wherein the actual position (X_(act)) of the slider (12) is determinedfrom at least one operating parameter (22 a to 22 h) of the actuator(28).
 24. A method in accordance with claim 23, wherein the operatingparameters (22 a to 22 h) for the determination of the position of theslider (12) is a parameter corresponding to the angle of rotation (22 d)of the electric motor (20).
 25. A method in accordance with claim 17,wherein, with a negative sign, the control algorithm (31) forms acontrol variable (34) for the electric motor by a comparison of thedesired gradient (dp/dt_(des)) with the actual gradient (dp/dt_(act)) ofthe pressure in the actuator cylinder (4), with the desired gradient(dp/dt_(des)) being formed as a function of the desired pressure(p_(des)) and of the actual pressure (p_(act)) in the actuator cylinder(4) and the actual gradient (dp/dt_(act)) being formed by timederivation of the actual pressure (p_(act)) in the actuator cylinder(4).
 26. A method in accordance with claim 17, wherein, with a negativesign, the control algorithm (31) is that of a cascade controller, with adesired speed (n_(des)) of the electric motor being determined in afirst sub-controller (63) from the desired position (x_(des)) and theactual position (x_(act)) of the slider (12), a desired electricparameter (I_(des)) being determined in a second sub-controller from thedesired speed (n_(des)) and the actual speed (n_(act)) of the electricmotor and a control parameter with which the electric motor (20) iscontrolled being determined in a third sub-controller from the desiredelectrical parameter (I_(des)) and the actual electrical parameter(I_(act)). (Cascade)
 27. A method in accordance with claim 17, wherein,when the sign of the difference of the desired pressure (p_(des)) andactual pressure (p_(act)) is equal to zero, the control algorithm (32)monitors the actual pressure (p_(act)) in the actuator cylinder andforms a control variable (34) for the electric motor (20) at a definedpressure drop.
 28. A method in accordance with claim 17, wherein, whenthe sign of the difference of the desired pressure (p_(des)) and actualpressure (p_(act)) is equal to zero, the control algorithm (32) monitorsthe position (x) of the slider (12) and forms a control variable (34)for the electric motor (20) on a defined deviation.
 29. A method inaccordance with claim 28, wherein the control parameter (34 c) for theelectric motor is the motor current or the motor voltage for the holdingclosed of the fast drain valve (8), with in particular the motor currentor the motor voltage being kept constant.
 30. A system (21) for thecontrol of a hydraulic actuator of a friction clutch (7), whichcomprises: a) a pump (19) driven by an electric motor (20) that iscontrolled by a control system (21); b) a pressure line (18), whichcontains a check valve (9) and which runs from the pump (19) to anactuator cylinder (4) having an actuator piston (5) acting on thefriction clutch (7), with a pressure (p) in the actuator cylinder (4) tobe controlled; and c) a fast drain valve (8), which is in flowcommunication with the actuator cylinder (4) and includes a slider (12)responsive to the pressure prevailing at the side of the pump (19)facing it; wherein the system includes a processor (26) and a driverstage (27) for the control of the electric motor (20), and wherein theprocessor (26) forms at least two controllers (30, 31; 32) withdifferent control behaviors and includes a selection logic (33), whichselects the output signal (34) of the one (30) or the other controller(31; 32) depending on whether the pressure (p) in the actuator cylinder(4) should be raised or lowered.
 31. A system in accordance with claim30, characterized in that at least one of the one (30) and the othercontroller (31) is made as a cascade controller, with, in the cascade, afirst sub-controller (40; 63) comparing the respective desired values(p_(des); x_(des)) and the respective actual values (p_(act); x_(act))with one another and forming a desired speed (n_(des)) for the electricmotor (20), a second sub-controller (43; 65) comparing the desired speed(n_(des)) of the electric motor with the actual speed (n_(act)) andforming a desired electrical parameter (I_(des)), and a thirdsub-controller comparing the desired electrical parameter (I_(des)) withan actual electrical parameter (I_(act)) and determining a controlparameter (34) with which the electric motor is controlled.
 32. Afriction coupling (7) for the drivetrain of a motor vehicle comprisingan actuator, which acts on the clutch disks and comprises: a) a pump(19) driven by an electric motor controlled by a control system; b) apressure line (18), which contains a check valve (9) and which runs fromthe pump (19) to an actuator cylinder (4) having an actuator piston (5)acting on the friction clutch (7), with a pressure (p) in the actuatorcylinder to be controlled; and c) a fast drain valve (8) which is inflow communication with the actuator cylinder (4) and includes a slider(12) responsive to the pressure prevailing at the side of the pump (19)facing it; and a control system (21) in accordance with claim 30, withthe maximum torque to be transmitted by the friction clutch (7) beingsubstantially proportional to the pressure (p) in the actuator cylinder(4).